Superjunction device with improved avalanche capability and breakdown voltage

ABSTRACT

A superjunction device has a plurality of equally spaced P columns in an N −  epitaxial layer. The concentration of the P type columns is made greater than that needed for maintaining charge balance in the N −  epi region and the P columns thereby to increase avalanche energy. An implant dose of 1.1E13 or greater is used to form the P columns.

RELATED APPLICATION

[0001] The application is based on and claims benefit of U.S.application Ser. No. 09/927,027, filed Aug. 9, 2001, entitledSuperjunction Device with Improved Avalanche Capability and BreakdownVoltage, to which a claim of priority is made.

FIELD OF THE INVENTION

[0002] This invention relates to semiconductor devices and morespecifically relates to a superjunction type power MOSFET with increasedavalanche energy.

BACKGROUND OF THE INVENTION

[0003] Superjunction power MOSFETs are well known. The static anddynamic characteristics of such devices are also described in “Analysisof the Effect of Charge Imbalance on the Static and dynamicCharacteristics of the Superjunction MOSFET by Proveen M. Shenoy, AnupBhalla and Gray M. Dolay, Proceeding of the ISPSD '99, pp. 99-102, June1999.

[0004] In such devices, the avalanche capability, sometimes called“ruggedness” is determined mainly by the means of preventing the turn onof the inherent parasitic bipolar transistor in a DMOS type MOSgateddevice. However, in the superjunction device, the concentration of the Ptype columns is chosen to maintain charge balance in the active area ofthe epitaxial silicon body material. This requirement lowers theavalanche capability of the device because the high field locates in theN type region of the epitaxial silicon layer, resulting in a higher baseresistance R_(b) ¹ in the avalanche current path through the N typeregion and to the N+ source. Thus, in some designs, avalanche energy,that is, the amount of energy which is produced in avalanche withoutfailure, has been as low as 50 millijoules. Attempts to increase thisenergy results in a reduction of the device breakdown voltage.

[0005] It would be desirable to increase the avalanche energy of asuperjunction device without degrading the breakdown voltage.

BRIEF DESCRIPTION OF THE INVENTION

[0006] In accordance with the invention, the P column dose in asuperjunction device is increased to a value intentionally higher thanthat required for charge balance in apparent disregard of the acceptedtheory and design rules for superjunction devices. By doing so, the highfield location moves from the N region to the P column, and, therefore,a lower R_(b) ¹ or lateral base resistance is experienced by theavalanche current through the P column to the source. Thus, theavalanche capability of the device is significantly improved (by afactor greater than 10) without degrading breakdown voltage.

[0007] For example, in a prior design using an N epi layer concentrationof 1.26E15 and a P column dose of about 1E13, the P column dose wasincreased to 1.1E13 and avalanche energy was increased from 50millijoules to 2500 millijoules. The P column dose to be used isdependent on die size and it was found that a higher dose can be used onsmaller area die. Thus, a P column dose of 1.2E13 was used for a die ofsize 110×140 mils; while a dose of 1.1E13 was used on larger die of257×330 mils and 315×450 mils. In all cases, a dose of 1.1E13 to 1.3E13can be used to improve avalanche capability without adversely affectingbreakdown voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a cross-section of a small area of a superjunction dieand is a cross-section of a portion of FIG. 2 taken across section line2-2 in FIG. 1.

[0009]FIG. 2 is a view of the top of the silicon in FIG. 1, taken acrosssection line 1-1 in FIG. 2 to show the topology of the P type pedestalsor channels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Referring to FIGS. 1 and 2, there is shown a silicon die havingan N⁺ body 10 which has an epitaxially deposited N⁻ top layer 11 formedthereon. Note that the term epitaxial layer means a layer of siliconwhich was grown by an epitaxial process. Layer 10 is about 500 micronsthick and layer 11 is about 17 microns thick for a 500 volt device. TheN⁻ concentration is typically about 1.26 atoms/cm³ (or about 3.5 ohmcm).

[0011] A plurality of spaced P columns 12 are formed in layer 11 asshown. P columns may have a depth of about 6.4 microns and acenter-to-center spacing of about 15 microns. The P columns are shown ashexagonal in section, but they can have any other shape and; if desired,may be rectangular, square, or even have a parallel elongated stripeform. When hexagonal as shown, the hexagonal columns may be 6.4 micronswide when measured perpendicular to parallel sides, and may be spaced byabout 8.6 microns from all adjacent columns.

[0012] The P columns may be fabricated by sequentially growing N⁻epitaxial layers about 6.4 microns thick and diffusing the hexagonal Players which are aligned with one another to build the full P column. Aswill be later emphasized, the P columns 10 are formed by implants ofboron at 80 KeV and at a dose of 1.1E13 or greater, but in accordancewith the invention, will have a value which is greater than that neededfor charge balance between the N⁻ epitaxial silicon 11 and the P columns12.

[0013] After forming the P columns 12, a gate oxide and a conductivepolysilicon lattice 13 (atop the gate oxide) is formed in the latticespace between columns 12. A shallow N⁺ source 14 is then formed into thetop of each column 12 as by implantation and diffusion, to definechannel regions under the gate 13. A thin P⁺ contact layer 15 is formedbeneath each source layer 14. The top of polysilicon gates 13 are cappedby an interlayer oxide 16 and a shallow trench is formed through sources14 and into P⁺ regions 15. An aluminum source electrode 17 is thenformed atop the upper surface of the die, making contact with sourceregions 14 and P⁺ regions 15. The top of source 17 is covered by asuitable passivation layer 18. A drain electrode 20 is formed on thebottom of die 10.

[0014] In accordance with the invention the concentration in the Pcolumn is greater than needed for charge balance to the surrounding N⁻epitaxial silicon. In particular and with an N⁻ region resistivity of3.5 ohm cm and the dimensions given, a P concentration defined by animplant dose of 1.0×10¹³ atoms/cm² would be used in the prior art. Inaccordance with the invention however, and for a die of 315×450 mils,the dose of 1.1E13 for columns 12 increased avalanche energy over 10fold. For a smaller die of 110×140 mils, a dose of 1.2×10¹³ atoms/cm²can be used with the same benefit.

[0015] Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein.

What is claimed is:
 1. A superjunction device comprising a flat thinmonocrystalline silicon die having an N⁺ body and a junction-receivingepitaxially grown^(—) layer on top of said N⁺ body; a MOSgate structureformed on the top surface of said^(—) epitaxial layer of silicon; saidMOSgate structure including a source region in the top of each of said Pcolumns and defining invertible channel regions in said P columns and agate structure spanning across said invertible channel regions andsaid^(—) epitaxial silicon; and a means for increasing the avalancheenergy of the superjunction device without affecting its breakdownvoltage, said means being comprised of a plurality of equally spaced Ptype columns of similar cross-section extending into said epitaxiallayer for a substantial portion of the thickness of said epitaxial layerand perpendicularly to the top and bottom surfaces of said die; said Ptype columns having a P type concentration which is intentionallygreater than that needed for charge balance between said P columns andthe surrounding^(—) epitaxial silicon.
 2. The device of claim 1, whichfurther includes a MOSgate structure formed on the top surface of saidN⁻ epitaxial layer of silicon.
 3. The device of claim 2, wherein saidMOSgate structure includes a source region in the top of each of said Pcolumns and defining invertible channel regions in said P columns and agate structure spanning across said invertible channel regions and saidN⁻ epitaxial silicon.
 4. The device of claim 1, wherein said P column isformed from boron implants having a dose of greater than about 1.1E13.5. The device of claim 2, wherein said P column is formed from boronimplants having a dose of greater than about 1.1E13.
 6. The device ofclaim 3, wherein said P column is formed from boron implants having adose of greater than about 1.1E13.